As a firefighter, I went to a total loss house fire started by a Power Wheels car next to the back of the house. Fire climbed the wall and it was all over from there. (the homeowners got home after the fire was called in to us by a neighbor)

Lithium batteries aren't safe anywhere in or near a house. (including tablets and phones) You just never know. With that said, yes, I leave my tablets/spare phones at home, but not on a charger.